Thermally controlled solar reflector facet with heat recovery

ABSTRACT

A high concentration central receiver system and method provides improved reflectors and a unique heat removal system. The central receiver has a plurality of interconnected reflectors coupled to a tower structure at a predetermined height above ground for reflecting solar radiation. A plurality of concentrators are disposed between the reflectors and the ground such that the concentrators receive reflective solar radiation from the reflectors. The central receiver system further includes a heat removal system for removing heat from the reflectors and an area immediately adjacent the concentrators. Each reflector includes a mirror, a facet, and an adhesive compound. The adhesive compound is disposed between the mirror and the facet such that the mirror is fixed to the facet under a compressive stress.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/879,363 filed on Jun. 12, 2001. The disclosure of the aboveapplication is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to solar power plants. More particularly,the invention relates to a high concentration central receiver systemhaving improved reflectors and a unique heat removal system.

As concerns over the environment, the deterioration of fuel sources, andenergy efficiency continue to increase, solar power plants have becomethe subject of worldwide attention. In the development of solar powerplants, high concentration central receiver systems have demonstrated arelatively high level of usefulness and are therefore quite popular. Theconventional solar central receiver system has a “tower top”configuration in which a field of heliostats reflect sunlight onto areceiver mounted on a tower structure. The concentrated solar energy onthe receiver heats a fluid, such as oil or molten salt, to hightemperatures. This energy is then transferred to a boiler/heat exchangerto produce steam, which then powers a steam turbine to produceelectricity. While this type of configuration has been shown to beuseful for power plants, other configurations have proven to be moreeffective for large power plants, especially when operated with highefficiency, combined cycle gas turbines powered by both natural gas andsolar energy.

One such configuration is the “tower reflector” configuration. One ofthe major features of this type of configuration is that specialparabolic concentrators are located on the ground beneath the tower andreflectors are coupled to the tower structure at a predetermined heightabove ground for reflecting solar radiation. The tower-mountedreflectors redirect sunlight from the heliostats, to the parabolicconcentrators which are located on the ground. The tower-mountedreflector is composed of a number of mirrors, coupled to a metallicfacet (or heat exchanger) for support. Each parabolic concentratortypically has a special quartz receiver into which the concentratedlight is directed. Air flowing through this receiver is air heated to ahigh temperature and then passes into the turbine combustion chamber,where it is further heated, before passing through the turbine toproduce electric power by turning the generator.

It is critical that the tower mounted reflectors provide the light intothe aperture opening of the parabolic concentrators at the appropriateangles under a wide variety of conditions. These conditions includetemperature changes, wind variations, solar insolation levels, sunangles, etc. It is very desirable that the concentrators have verylittle loss due to “spillage” under these conditions, becauseconventional systems make no use of this wasted heat. The towerreflectors must therefore achieve high optical quality at a low cost.The tower reflectors must also be able to withstand high concentrationsof solar energy and meet the optical requirements under a wide varietyof environmental conditions. It is therefore desirable to providereflectors having a good structural integrity and that are safe tooperate. It is also desirable to enable the reflectors to be adjustableand configurable such that there is minimal loss of reflected light fromthe heliostats in harsh environments and over several decades.

A particularly difficult aspect of conventional solar reflectors relatesto high operating temperatures, cost and breakage. Specifically, whilevarious facet designs and heat removal systems have been designed fortower reflectors, a number of difficulties remain. For example, theconventional design has a small reflector area and uses small, hightensile strength, thick glass mirrors. Generally, these mirrors havebeen shown to be too costly for practical use in high temperaturecommercial applications. The conventional design is also prone tobreakage, since the glass is held by “clips” such that there are slightstresses built up in the glass under nominal conditions. It is thereforeeasy to understand that such systems can impose relatively high levelsof stress at local points under more severe conditions. For example,high stresses occur (especially when exposed to sand, dust and ice,since these can cause “ratcheting”) when the glass expands and contractsdue to exposure to diurnal cycles of high concentration irradiance, withhigh temperatures, followed by little or no irradiance and relativelycool temperatures.

The resultant expansion and contraction, with metal joints used to holdthe glass securely for good alignment, can result in high local stressesand breakage. Since the glass is not otherwise constrained, it can fall,causing a significant hazard to equipment and personnel below. Inparticular, the falling glass can damage the high optical quality,relatively high cost Compound Parabolic Concentrators (CPCS) on theground. These thermal and stress related problems are exacerbatedfurther by the exposed clips, which can be subjected to over 50 to 100suns (i.e., 50 to 100 kW/m²). Since the metal has a relatively highsolar absorptivity, the operating temperature of the metal clips can bequite high, thus adding to the local thermal stresses already placed onthe facets by the direct, concentrated solar flux.

The conventional approach also does not provide for adequate thermalcontrol to prevent ice buildup. Ice buildup on high structures is aserious problem, since it can greatly increase the structural load,distort and damage the glass mirrors. If ice forms and falls onto theCPCs, further damage is likely to be caused to the system. It istherefore desirable to provide a design that ensures thermal control toprevent buildup.

Another aspect of the conventional design is that it uses a rectilinearsupport structure. Such supports do not offer the torsional stiffnessinherent in geometries such as triangular shapes. The mass of materialrequired, and the complexity of assembly (as well as cost) are thereforehigher than for other geometric shapes. For example, the triangulardesign disclosed herein, is formed with a novel “geodesic dome” concept,that uses essentially equilateral struts arranged with novel attachmentfittings to allow easy assembly of the support structure and adjustmentof the facets.

In the more general case, for certain applications, mirrors are heatedby incident solar irradiance and/or heat flux. This heating can causedamage to the mirrors or to the support backing structure. Furthermore,the optical quality can be degraded by changes in the radius ofcurvature, increases in the surface slope error, damage to thereflective surface, or warpage. The problem is typically solved byeither selecting high tensile strength glass (at high cost), flowing astream of air over the mirror, or using a fluid coolant. It is importantto note that while conventional coolant-based heat removal systems aremoderately effective in sinking heat away from the reflectors, othershortcomings remain. For example, the “spillage” area immediatelyadjacent the concentrators is also a considerable source of heat.Removing heat from this area would both improve the operation of theconcentrators as well as provide additional heat to other systems (e.g.,residential/commercial systems). As already mentioned, extreme cold orice buildup can also cause problems. These problems include warpage ofthe facet or its support structure, changes in the facet cant angle,build-up of extremely high loads on the structure, or cracks in theglass. To mitigate concerns of extreme cold and ice buildup, anembodiment of the invention utilizing a fluid coolant heat recoverysystem can maintain adequate coolant temperatures to prevent formationof ice and protect the area from extreme cold.

In general, the mirrors must be adjusted to produce the beam positioningrequired by the application. This problem is typically solved byattaching multiple (most often, three) adjustable attachment fittings tothe back of the mirror assembly. Also, for certain applications, themirror assembly must be very light weight, and in some applications themirror must be mounted in a location where access is difficult. Forexample, in the tower reflector case, the reflectors are mounted highabove the ground (hundreds of feet high). Therefore, the reflectors mustdemonstrate exceptional long life and integrity, while at the same timebeing light weight and inexpensive.

SUMMARY OF THE INVENTION

The above and other objectives are provided by a method and highconcentration central receiver system in accordance with the presentinvention. The central receiver system has a plurality of interconnectedreflectors coupled to a tower structure at a predetermined height aboveground for reflecting solar radiation. A plurality of parabolicconcentrators are disposed between the reflectors and the ground suchthat the concentrators receive reflected solar radiation from thereflectors. The central receiver system further includes a heat removalsystem for removing heat from the tower-mounted reflectors and an areaimmediately adjacent the parabolic concentrators. Removing the heat fromthe area immediately adjacent the concentrators improves operation ofthe concentrators and provides an additional source of energy that iseffectively wasted in conventional systems.

Further in accordance with the present invention, a reflector for a highconcentration central receiver system is provided. The reflectorincludes a mirror, a facet, and an adhesive compound. The facet haswalls defining a coolant channel, where the cooling channel receives aheat conductive fluid. The adhesive compound is disposed between themirror and the facet such that the mirror is fixed to the facet undercompressive stresses. In the preferred embodiment, the glass mirror hasa compression stress value such that no part of the glass experiencestensile stresses. Generating compressive stresses in the mirror improvesthe strength and resistance to breakage because glass has a low tensilestrength. Pre-loading the glass in compression thus avoids the mostcommon failure mode for glass.

In another aspect of the invention, a method for fabricating a reflectorfor a high concentration central receiver system is provided. The methodincludes the step of maintaining a mirror at a mirror bondingtemperature. A metal facet is maintained at a facet bonding temperature,where the facet bonding temperature is greater than the mirror bondingtemperature. The method further includes the step of bonding the mirrorto the facet with an adhesive, where an operating temperature for thereflector is less than the facet bonding temperature. Due to theinherent properties of glass and metal, the mirror is under compressivestresses at the operating temperature.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed. The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitutepart of this specification. The drawings illustrate various features andembodiments of the invention, and together with the description serve toexplain the principles and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The various advantages of the present invention will become apparent toone of ordinary skill in the art by reading the following specificationand sub-joined claims and by referencing the following drawings, inwhich:

FIG. 1 is a diagram of a tower structure with a high concentrationcentral receiver system according to the present invention;

FIG. 2 is a diagram of a heat removal system in accordance with theprinciples of the present invention;

FIG. 3A is a top view of a reflector according to one embodiment of thepresent invention;

FIG. 3B is a side view of the reflector shown in FIG. 3A;

FIG. 3C is a cross sectional view taken along lines 3 c-3 c in FIG. 3A;

FIG. 4A is a top view of a reflector having cooling fins according to analternative embodiment of the present invention;

FIG. 4B is a side view of the reflector shown in FIG. 4A;

FIG. 5A is an end view of a reflector having cooling fins according toan alternative embodiment of the present invention;

FIG. 5B is a cross sectional view taken along lines 5B-5B shown in FIG.5A;

FIG. 6A is a cutaway top view of a reflector showing three embodimentsof a turbulence generating system according to the present invention;

FIG. 6B is a cross sectional view taken along lines 6B-6B shown in FIG.6A;

FIG. 6C is a cross sectional view taken along lines 6C-6C shown in FIG.6A;

FIG. 6D is a cross sectional view taken along lines 6D-6D shown in FIG.6A;

FIG. 7 is a top view of a reflector having facet walls that define aplurality of channels according to an alternative embodiment of thepresent invention;

FIG. 8A is a top view of a plurality of interconnected reflectorswherein each reflector has facet walls that define a plurality ofcoolant channels according to an alternative embodiment of the presentinvention;

FIG. 8B is a diagram showing coolant flow paths for the configurationshown in FIG. 8A;

FIG. 9 is a side view of a plurality of reflectors, where the reflectorshave honeycomb shaped stiffening plates;

FIG. 10 is a side view showing attachment fittings according to oneembodiment of the present invention; and

FIG. 11 is a side view showing attachment fittings according to analternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numeralswill be used throughout the drawings to refer to the same or like parts.

Turning now to FIG. 1, a high concentration central receiver system 20is shown in greater detail. The receiver system 20 has a plurality ofinterconnected reflectors 22 (or reflector assemblies) coupled to atower structure 24 at a predetermined height above ground for reflectingsolar radiation 26. A plurality of concentrators 28 are disposed betweenthe reflectors 22 and the ground such that the concentrators 28 receivereflected solar radiation from the reflectors 22. A heat removal system30 (or spillage collector) removes heat from the reflectors 22 and anarea immediately adjacent the concentrators 28.

Thus, FIG. 1 shows the basic beam down optics central receiver conceptwith the reflectors 22 located atop a high tower structure 24 (of theorder of several hundred feet). The solar irradiance is concentrated onthe reflectors 22 from a field of heliostats 29. For a 10 Megawatt(thermal) system, approximately 1300 heliostats, each approximately 9 to10 square meters in area, are needed. The solar flux, or irradiance,incident on the tower can easily range up to approximately 50 kW/m², orhigher, which could cause high temperatures (several hundred degrees F.in the glass and higher in any exposed support structure). For thisreason, the mirrors of the reflectors 22 are cooled to prevent changesin optical properties, warpage, breakage, or separation of the glass andsupporting structure. The heat removal system 30 also prevents otherforms of damage and degradation, such as overheating of the supportstructure, loss of silver, accelerated corrosion at high temperature,deposition of foreign materials (with subsequent hot spots caused by theincident highly concentrated solar flux), etc. The mirrors are alsooverlapped, to minimize concentrated sunlight from overheating thesupport structure behind the mirrors.

FIG. 2 shows the heat removal system 116 block diagram. A heat transferfluid, such as water, mixed with an anti-freeze (e.g., propyleneglycol), is contained in a tank, 120. A pump 122 pumps the liquidthrough a pipe to the tower reflector structure where it passes througheach of the mirror assemblies 118. The temperature is raised frominitial inlet temperatures in the tank of the order of 10 to 30° C. tooutlet temperatures of the order of 50 to 90° C. The heat transfer fluidthen flows through a pipe down to the “spillage collector” 124 thatsurrounds the compound parabolic concentrators 126. The fluidtemperature is further increased, to temperatures of the order of100-120° C. The heat transfer fluid then passes through an optional heatexchanger capable of transferring additional waste heat and furtherraising the temperature. The heat transfer fluid then passes through aheat exchanger suitable for the selected end-use of the collected wasteheat. For example, the waste heat can be used to heat and/or desalinatewater for residential or industrial use. It can also be used in anorganic Rankine cycle (ORC) turbine generator, similar to geothermalheat recovery, to produce electricity. The heat transfer fluid exits theheat exchanger at a temperature on the order of 40 to 60° C. and can befurther cooled by passing through coils submerged in a cooling pond orin a cooling tower prior to entering the holding tank 120.

Although a series flow path is shown, other flow configurations can beused. For example, the flow out of the pump can be split such that partflows directly to the spillage collector 124 and part flows to the towerreflector 118.

It should be noted that there are substantial pressure differences dueto the hydrostatic head. The pressure through the mirror assemblies isrelatively low, compared to the pressure at ground level. This providestwo advantages. First, the lower pressure in the mirror assembliesminimizes distortion of the mirrors. However, the distortion that occursessentially causes the mirror to form a slightly convex mirror surface,as seen from below. This convex mirror surface is closer to the idealhyperbolic shape, and this improves the mirror optical quality.

Second, the higher pressure at ground level allows the heat transferfluid to remain a liquid or a two-phase mixture at moderately hightemperatures, thus improving heat transfer. As long as the two-phaseflow remains in the bubbly and slug flow regimes, the annular flowregime, or the initial region of annular to mist transition, the heattransfer coefficient will be high. However, as the fluid becomes a mistflow or forced convection vapor flow, the heat transfer coefficientdrops substantially. The lower heat transfer coefficient increases therequired temperature difference in heat exchanger size and cost and isthus to be avoided. The higher pressure helps keep the heat transferfluid in the high heat transfer coefficient region.

Turning now to FIGS. 3A and 3B, it can be seen that in one embodimenteach reflector 34 includes a mirror 36 and a facet 38 having walls 40defining a coolant channel 41. The coolant channel 41 is connected toone or more adjacent reflectors such that the adjacent reflectorsreceive a heat conductive fluid from the heat removal system and passthe fluid through the channel. As shown in FIG. 3C, an adhesive compound37 is disposed between the mirror 36 and the facet 38 such that themirror 36 is fixed to the facet 38 under a compressive stress. FIGS. 3Aand 3B show two views of a reflector 34 having a single pass coolantfluid path. Although the preferred design has a triangular geometry, thepresent invention is not constrained to this shape. Other shapes (e.g.,rectangular, hexagonal, circular, or square) may therefore be usedwithout parting from the spirit and scope of the invention.

As already noted, the mirror 36 is bonded to the facet 38 (or backingplate). Note that the mirror 36 may be glass or glass laminated to aprotective substrate (sheet steel or plastic) and may be one piece orseveral pieces. Mirrors with the silver reflector coating on the backside of the glass and with a protective substrate are commonly referredto in the industry as second surface mirrors. This assembly is held in afixture during the adhesive curing cycle to obtain the required finishedcontour (e.g., flat, cylindrical, spherical, parabolic, or hyperbolic).The portion of the drawing in FIG. 3B shows the flow path that is formedby the facet design. The facet 38 is embossed, stamped, or hydroformedmetal or reinforced plastic (match die molded, or spray/hand lay-up) toyield the cavities for the cooling media to pass through. Note thatreference is made to “cooling” where, as will be explained below,certain applications require a heated fluid.

It is also important to note that FIGS. 3A and 3B show simple “L” shapedfluid fittings 42, which may be welded, brazed, or bonded, etc. to thefacet 38, depending on the application and materials selected. Forsupport, three attachment fittings 44 are secured to the facet 38. Whilethe illustrated attachment fittings 44 are simply threaded studs thatmay be welded, brazed or bonded to the facet 38, other options will bedetailed later.

It will be appreciated that the flow path may be designed in severaldifferent ways to achieve the desired effect. For example, FIGS. 3A and3B show a simple, single pass approach. The depth and width of thecavity should be optimized to supply the necessary cooling by providingthe required fluid velocity and pressure drop for the system (especiallywhen a large number of mirror facets 38 are connected in series).Modifications to this simple configuration will be described below.

FIG. 4A and FIG. 4B show the same simple, single pass facet 38 as shownin FIGS. 3A and 3B, with cooling fins 46 attached. This design offersthe additional advantage of ensuring a uniform temperature of the mirror36, due to the coolant flow, while rejecting the heat to the atmosphere.This approach could be used when it is not necessary to recover thewaste heat. The cooling fins 46 of this design would preferentially bemade from metal in order to have good heat transfer, but plasticmaterials (especially with good thermal conductivity) are alsopermissible. For facets 38 that are formed from metal, welding orbrazing of the cooling fins 46 would provide efficient heat transfer.For applications that are weight dependent, the fins undesirably addweight to the assembly, but they offer very good stiffness advantages.Therefore the facet 38 may be made thinner while maintaining equivalentfacet stiffness.

FIG. 5A and FIG. 5B illustrate detailed views of the preferred coolingfin design. For facets made of steel, stainless steel, aluminum, copper,etc., spot welding or roll-spot welding of the cooling fins 46 aresimple and efficient methods for providing good heat transfer across thejunctions 48. Welding also provides adequate strength to maintain thestiffening qualities of the assembly. Bonding with a thermallyconnecting adhesive is also a viable option.

To provide the coolant, the mirrors 36 are connected to a fluid coolingloop via fittings on a back surface 36 a. Each mirror facet 38 has acoolant path provided by an embossed or ribbed structure which providesthe flow channels required. The configuration of the flow can be singleor double pass, but other options are possible. This supportingstructure for the coolant flow may be formed sheet metal (steel,stainless steel, aluminum, etc.) or a plastic material (sheet moldingcompound, spray or hand layup fiberglass, graphite/epoxy, etc.).

It should be noted, however, that the best combination of glass andfacet from the standpoint of reducing the tensile stress in the glass isthe selection of materials that have similar coefficients of thermalexpansion (CTE) to minimize tensile stress. Alternatively, for thesituation in which compressive loads are imposed on the glass 36, thefacet 38 should preferentially have a CTE slightly greater than that ofthe glass. Typically, fiber glass or steel comes closest to matching thecoefficient of thermal expansion of the various types of glass that arecandidates for this design, and both of these have CTEs slightly higherthan glass. Aluminum has CTE significantly higher than glass, but caremust be taken to ensure that the stresses induced do not cause excessivedeformation. Other reflective surfaces are potential candidates, but dueto the lack of long life and low cost materials, the high reflectivityof silvered glass is preferred. Thus, although the preferred embodimentuses either steel, aluminum, or fiberglass for the facet 38, the presentinvention is not restricted to these materials.

Another aspect of the design that reduces the stress on the glass 36 isthe use of an adhesive that has the correct combination of complianceand thermal conductivity. In fact, various adhesives satisfy thesecriteria, as specified further below. The glass 36 can also be bonded toa thin, protective sheet metal or composite material having a goodthermal conductivity characteristic, using a double backed sheetadhesive. Alternatively, the adhesive can be applied directly to theglass back surface and/or to the facet by various well known techniques(spray, curtain coat, roller coat, brush, etc.). The glass mirror 36without the protective laminate layer is then bonded (with a thermallyconductive adhesive) to the triangular facet 38 that provides thecoolant loop for thermal control.

Regardless of the material selected for the facet 38, the internalcoolant loop flow path is designed to be large enough in cross sectionalarea, short enough in path length, and the flow rate selected to be lowenough such that flow resistance is not excessive. At the same time thefacet 38 facilitates maintaining conditions for effective heat transferto control the temperature of the glass, and the resultant stresses, toacceptable levels. In particular, the glass 36 temperature is uniformacross its surface to minimize stress concentrations, and is maintainedat temperatures of approximately 50 to 100 degrees Centigrade, or lower.Various means may be employed to enhance the heat transfer.

FIGS. 6A-6D illustrate three different methods of adding turbulentgenerating features to the design to further enhance heat transfer. Heattransfer is greatly improved when the coolant is in a state of turbulentflow. For this reason, some means of disturbing the laminar flowconditions may be required for a specific application, especially in thevicinity of the thermally conducting fins. The upper example of FIG. 6Bshows how a molded facet 50 could have inner fins 52 formed on theinterior to force the fluid into a turbulent state. Note that thisincreases the pressure drop (and therefore the need for a stronger paneldesign). The fins 52 may be contoured such that the base 52 a of eachfin 52 stiffens the facet 50, but the tip 52 b is flexible enough to“flutter” and cause additional turbulence, especially in the vicinity ofthe inner surface of the heated mirror 54.

Where a mirror 56 is bonded to a metal substrate 58, as shown in thelower left example of the FIG. 6C (thence bonded to a facet 60), thesubstrate 58 may be formed with inner projections 62 to alternate withsimilar deformations 64 in the facet 60. The lower right hand portion ofFIG. 6D shows how a facet 66 may be formed to have a serpentine path ofthe fluid as it passes around projections 68 from formed dividers 70 ofthe facet 66.

FIG. 7 shows a reflector 72 having a two-path flow for the fluid. Thisapproach has two particularly important advantages. First, this flowconfiguration tends to better average the facet temperature. Secondly,it allows the inlet and exit fluid fittings 74 to be inline. Althoughthis is of little advantage for one facet, the facet interconnectbecomes much simpler for a group of facets as seen in FIG. 8A to follow.

FIG. 8A and FIG. 8B illustrate how multiple reflectors 72 of thetwo-path flow type may be connected to achieve efficient cooling of aportion of a large facet array 76. With this configuration, a simpleplastic or rubber tube may be used as a fluid fitting 74 for low tomoderate pressure systems. This resilience allows for adjustment of theindividual facets (via various attachment fittings) of the array 76without straining the reflectors. The insert sketch of FIG. 8B shows asimplified flow pattern 78 for this type of interconnect.

FIG. 9 shows a facet design with a honeycomb-shaped stiffening plate 80supporting a mirror 82. The plate 80 increases the stiffness such thathigher pressure drops across facets 84 can be accommodated withoutcausing the mirror 82 to deform in a short radius of curvature convexshape (looking at the mirror) that could tend to spread the reflectedbeam by an excessive amount.

FIG. 10 shows a variation on the simple attachment design shown in FIGS.3 and 9 for which the attachment fitting 44 is a simple, threaded studthat forms a coolant feed fitting 90. Here an adjusting mechanism 86 isalso used for the fluid inlet/exit paths. A facet 88 has the coolantfeed fitting 90 welded, brazed, or bonded to one of a plurality ofraised flow paths 92. The flow paths 92 should be positioned so thatthey are parallel to each other to minimize alignment problems at thetime of the installation. A pair of lock nuts 94 and two sets ofspherical washers 96 are used to attach the facet 88 to a supportstructure 98. Preferably, the structure 98 has an oversized hole toallow for alignment variations. The coolant feed fitting 90 shown is astraight threaded tube, but other designs may be used for specificapplications. For example, a ball stud can be used to accommodate largeangular variations. Since a triangular facet will typically requirethree points of support (to minimize distortion of the mirror), one ofthe coolant feed fittings 90 could be a “dummy” and not be drilledthrough, or formed from a tube, for a flow passage as the other two are.However, there are other flow configurations that can benefit fromhaving one or two inlets with a corresponding two or one outlets. Thisis especially true at the outer periphery of the set of facets, wherethe flow enters an outer annular ring.

FIG. 11 illustrates a variation to the above embodiments. Here a fitting100 is welded, brazed, or bonded to a facet 102. The design is similarto an AN fluid fitting except that the normal 37 degree flared end hasbeen modified to have a spherical end. A swivel feed line 106 ismachined to have a matching spherical (concave) end. A sealing washereffects a leak tight joint while allowing for a minimal amount ofvariation of alignment. A matching spherical surface is machined on theback side of the flange of the swivel feed line 106. The standard “BNut” secures the joint when tightened but allows for movement whenloosened. Lock nuts 110 secure the reflector 112 to a support structure114. Note that spherical washers are not required for this design. Also,the tubing sleeve that is normally used with AN fittings, is not neededfor this design.

It should further be noted that this design provides several effectsthat mitigate the glass tensile stress, when properly used. First, theadhesive between the glass 36 and the protective backing (optional) canbe sufficiently compliant such that the difference in coefficient ofthermal expansion (CTE) does not induce as high a tensile stress as forthe case of intimate contact. Second, the adhesive used to bond theglass-protective sheet laminate to the triangular facet 38 (or heatexchanger) adds a further degree of compliance that reduces the stressimposed on the glass. Third, the adhesive(s) and the protective sheethave a temperature drop such that the facet 38 is at a lower temperaturethan the mirror 36, for incident solar energy, which tends to reducestresses, as will be explained further below. Fourth, the adhesiveand/or protective sheet thermal conductivity can be selected to providethe preferential temperature gradient properties. Stress in the glasscan also be minimized by proper selection of the adhesives and optionalprotective sheet, for a variety of glass and triangular facet thermalcoefficient of expansion characteristics.

Another innovation of the present invention that further improves theoptical performance, structural integrity, life and overall costeffectiveness, lies in the method of forming the completed facet so asto build compressive stresses into the mirror, especially for mirrorshaving glass that is exposed to high concentrations of solar energy orhigh loads (i.e., wind). Since the facets may be exposed to high solarirradiance, internal pressure, and temperature, all of which can inducetensile stress in the glass, facet bonding is performed at an elevatedtemperature of the steel, while controlling the temperature of the glassto be less than that of the steel. The CTE of steel typically exceedsthat of the type of glass applicable to mirrors of interest. Therefore,the steel is heated while cooling the vacuum table/mandrel which is incontact with the glass. The shape of the vacuum table/mandrel can beflat, concave or convex, depending on the required shape of the mirror.Heat applied to the steel accelerates the rate of curing of the adhesivein contact with the steel, which decreases the fabrication time and thusreduces cost. The high temperature bonding and subsequent cooling to theoperational temperature range (typically, −20 degrees F. to 120 degreesF.) ensures that there is little or no tensile stress induced in theglass. The same process can be used with aluminum and other metals, asdesired.

Minimizing the temperature difference between the glass and thetriangular support structure also helps to avoid stress buildup. This isaccomplished by using an adhesive, with the proper thickness, that has ahigh conductivity, usually obtained by loading it with aluminum or othermaterials. The proper selection of adhesive, glass, and supportstructure material can be accomplished such that the glass tensilestress (and the delaminating stress on the adhesive bond) are wellwithin the requirements. For example, glass typically can be stressed intension practically indefinitely at levels of the order of 500 psi orless. The normal breaking tensile stress for glass is approximately 2000to 3000 psi, or higher, depending on the type of glass. It is thereforeimportant to minimize tensile stress, or preferably, eliminate it byplacing the glass in compression as disclosed above.

It is also important that the glass conduct the heat to the facet/heatexchanger, such that the waste heat in the thermal fluid can berecovered and used in power generation, process heat, space heating,etc. Accordingly, the appropriate combination of materials having therequired thicknesses, compliance, thermal conductivity, and strength areused to meet these combinations of appropriate compliance, thickness,and thermal conductivity. Since steel has a thermal coefficient ofexpansion (CTE) close to, but higher than that of glass, and since theglass is at a temperature slightly higher than the steel, there is atendency for the glass and steel to expand at approximately the samerate. Specifically, assume that Tf is the temperature at which the glassand steel are bonded together (both glass and steel are at the sametemperature during this process), Tog is the operating temperature ofthe glass, Tos is the operating temperature of the steel, and CTEg andCTEs are the coefficients of thermal expansion of the glass and steelrespectively. Then, when the glass and steel are operated at atemperature different from the temperature at which they were bondedtogether, there will be some stresses set up in the glass and steel.This stress difference is proportional to the difference in the productof CTEs and temperature difference (Tos-Tf) and the product CTEg andtemperature difference (Tog-Tf). At sufficiently high tensile stressvalues, the glass would break. But, since CTEg is less than CTEs, andTog-Tf is typically greater than Tos-Tf, there is a tendency for thetensile stress to be reduced or eliminated.

The method of forming the facet with the built in compressive stress inthe glass can be accomplished several ways according to the presentinvention. Specifically, under one approach the steel is maintained at ahigher temperature than the glass during bonding. For example, a heatedfluid, heat lamps, electrical heaters, or other such means can be usedto heat the steel facet/heat exchanger. Conversely, the glass is cooledby the form on which it is placed. This form, or mandrel, maintains acertain curvature or flatness (e.g., a vacuum table or other surfacehaving a cooling fluid, refrigerating coils). The steel can be at atemperature of, say, 100 degrees Centigrade—well above its operationaltemperature in the field —while the surface in contact with the glasscan be maintained at a temperature less than, or equal to, roomtemperature. Thus, when the steel/glass laminate is bonded together andthen removed from the bonding table, the steel would tend to contractfar more than the thin glass (typically, of the order of 1 mm), thusputting the glass in compression.

In addition, if the glass were maintained at a temperature less than itsoperating temperature during the bonding process, then the glass wouldtend to expand, but being constrained by the steel, would encounteradditional compressive stresses. This latter effect, however, thoughbeneficial from a stress standpoint, could decrease the rate at whichthe adhesive is cured depending on the type of adhesive and its curerate vs. temperature properties. On the other hand, there are manyadhesives for which this would not be a problem, and high cure ratescould be achieved even at temperatures well below room temperature.Since glass in compression is approximately as strong as steel, thecompressed glass would have greatly improved integrity and life.

It is important to note that the adhesive bond is preferably fullycured, since otherwise the compliance and “flow” of the adhesive wouldtend to reduce the compressive stress in the glass. It is also preferredto have the bonding process completed as quickly as practical to have ahigh production rate from the tool to reduce costs. Therefore, foradhesives requiring elevated temperatures for rapid curing, the glass isnot cooled to a low temperature, since this tends to lower the rate atwhich the adhesive bond is cured, especially close to the glassinterface. Rather, the steel is heated to a temperature above themaximum operating temperature, but well below the acceptable operatingtemperature for the adhesive. Again, the preferable approach is to havethe adhesive formulated for rapid cure even at temperatures at, or wellbelow, room temperature. Rapid curing allows significant temperaturedifferences between the glass and steel, which in turn allows greatercompressive loads to be imposed on the glass.

By minimizing the tensile stresses, or preferably, imposing compressivestresses in the glass, the fluid can be used at a higher temperature andat a higher pressure to cool the mirrors. The higher fluid temperatureand higher pressure tends to cause the glass to bow out, forming aconvex shape. By being able to operate the mirrors at a highertemperature, higher efficiency is achieved in the power conversionsystem (e.g., Organic Rankine Cycle turbine) or greater benefits areachieved for process heat or space heating applications. By being ableto operate at a higher pressure, higher flow rates can also be achieved.Thus the height of the channels is diminished, less coolant is used, andthe weight of the facet with the coolant is reduced—all of which reducescosts.

It should also be noted that the shape of the mirror facet is preferablytriangular. This is a key feature of the preferred embodiment. Thetriangular shape allows the design to be used with a “geodesic dome”supporting structure. A novel modification has been made to the usualapproach for joining the struts that form the geodesic shape needed tohave the right optical shape (usually, a hyperboloid). This design has aplate which allows the facets to be affixed to the plate by adjustmentscrews or fixtures. The plate is sized such that the facets can beoverlapped, to maximize the reflected energy to the receiver below thetower mounted reflector, and to minimize exposure of the tower structurebehind the facets to intense radiation from the heliostats below.

Returning now to FIG. 1, the use of the facet in a representative systemapplication is highlighted as follows. In this case, approximately 450square meters of reflectors 22 are arranged to form a hyperboloidalshape, located on top of the tower structure 24. The reflectors redirectenergy delivered from a field of heliostats 32 (approximately 1300heliostats of approximately 10 square meters area in this example) backto ground level. This redirected energy will deliver approximately 10Megawatts of thermal energy (for the specific field design selected),which is then transformed into approximately 3 to 5 Megawatts ofelectrical power by the through conversion in a gas turbine or combinedcycle system, using for example, a steam turbine and Organic Rankinebottoming cycle.

There are various uses of the waste heat collected at the towerreflector, which is at a temperature of the order of 50 to 100 degreesCentigrade. These uses include but are not limited to industrial processheating, space heating, or conversion to additional electric power in alow temperature Organic Rankine cycle turbine, as is commonly done withgeothermal plants and in certain co-generation applications.

In particular, conversion of waste heat to additional electric powerprovides unique economic benefits. For example, the peak flux on thefacets is typically of the order of 50 kW/m² or higher, and the averageflux is of the order of 5 to 10 kW/m². A pump, located on the ground,delivers a water/ethylene glycol (or equivalent) solution to the top ofthe tower structure 24. The heat transfer fluid then splits into acombination of series and parallel flows through the reflector facets insuch a fashion as to maintain the facet operating temperature atapproximately 80 to 100 degrees C. while keeping the pressure drop to apreferred level.

The outlet of the reflectors 22 is returned to ground level to interfacewith the Organic Rankine bottoming cycle via a heat exchanger. Theadditional power generated in the bottoming cycle from use of the pumpedloop cooled facets generates sufficient cash flow to pay for the activethermal management system and the facets. In this case, we have assumeda relatively low price for the electricity sales of $0.10/kilowatt hour.Therefore, in markets where solar electrical power is priced at valuesof the order of $0.10/kilowatt hour or above (i.e., so-called GreenPower Pricing markets, or in subsidized or Portfolio Standards markets)we can expect that the thermally cooled facet design disclosed hereinwill more than pay for the entire tower reflector concept through wasteheat recovery.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, specification and following claims.

1. A reflector for a central receiver system, the reflector comprising:a mirror having a front surface and a back surface; a facet having wallsdefining a coolant channel, the coolant channel adapted to receive aheat conductive liquid wherein the facet includes a metal backingcoupled to a metal substrate of the mirror, the metal backing and themetal substrate having alternating projections extending into thecoolant flow channel; and an adhesive compound bond between the backsurface of the mirror and the facet such that the mirror is fixed to thefacet and the facet imposes a thermally induced compressive stress alongthe back surface of the mirror; wherein the compressive stress imposedby the facet along the back surface of the mirror depends upon thermalcontraction of the facet and is sufficient in magnitude so that tensilestress on the mirror is minimized when the mirror is being operated atan operating temperature.
 2. The reflector of claim 1, wherein the facetincludes cooling fins for removing heat from the heat conductive liquid.3. The reflector of claim 1, wherein the reflector further includes astiffening plate disposed between the mirror and the facet.
 4. Thereflector of claim 3, wherein the stiffening plate comprises a honeycombshape.
 5. The reflector of claim 1, wherein the reflector comprises atriangular shape.
 6. The reflector of claim 1, wherein the facetincludes a turbulence generating system for disturbing laminar flow ofthe heat conductive liquid.
 7. The reflector of claim 1, wherein thefacet includes a molded backing coupled to the mirror, the moldedbacking having a plurality of fins extending into the coolant flowchannel.
 8. The reflector of claim 7, wherein the molded backingincludes reinforced plastic.
 9. The reflector of claim 1, wherein thewalls of the facet defining the coolant flow channel include a pluralityof alternating projections extending into the coolant flow channel. 10.The reflector of claim 1, further comprising a fluid coupling fittingcoupled to the facet for coupling the facet to an adjacent reflector.11. A reflector for a central receiver system, the reflector comprising:a mirror having a front surface and a metal substrate located at a backsurface of the mirror; a facet having a metal backing defining a coolantchannel, the coolant channel adapted to receive a heat conductiveliquid; and a thermally conductive adhesive compound bond disposedbetween the back surface of the mirror and the facet such that themirror is fixed to the facet and the facet imposes a thermally inducedcompressive stress along the back surface of the mirror; wherein themetal substrate and the metal backing have a plurality of alternatingprojections extending into the coolant channel; and wherein thecompressive stress imposed by the facet along the back surface of themirror depends upon thermal contraction of the facet and is sufficientin magnitude so that tensile stress on the mirror is minimized when themirror is being operated at an operating temperature.
 12. The reflectorof claim 11, wherein the metal backing of the facet comprises aserpentine fluid flow path.
 13. The reflector of claim 11, wherein thefacet has walls defining the coolant channel.
 14. The reflector of claim11, wherein the reflector comprises a triangular shape.
 15. Thereflector of claim 11, wherein the facet comprises at least one fluidflow fitting for coupling the facet in fluid flow communication with atleast one adjacently positioned reflector.
 16. A reflector for a centralreceiver system, the reflector comprising: a mirror having a frontsurface and a back surface; a facet having walls defining a coolantchannel, the coolant channel adapted to receive a heat conductive liquidwherein the facet includes a metal backing coupled to a metal substrateof the mirror, the metal backing and the metal substrate havingalternating projections extending into the coolant flow channel; astiffening plate disposed between the back surface of the mirror and thefacet for supporting the mirror; and a thermally conductive adhesivecompound bond disposed between the back surface of the mirror and thefacet such that the mirror is fixed to the facet and the facet imposes athermally induced built-in compressive stress along the back surface ofthe mirror; wherein the compressive stress imposed by the facet alongthe back surface of the mirror depends upon thermal contraction of thefacet and is sufficient in magnitude so that tensile stress on themirror is minimized while the mirror is being operated at an operatingtemperature.
 17. The reflector of claim 16, wherein the stiffening platecomprises a honeycomb shape.
 18. The reflector of claim 16, wherein thereflector comprises a triangular shape.
 19. A reflector for a centralreceiver system, the reflector comprising: a triangular mirror having afront surface and a back surface; a triangular facet having wallsdefining a coolant channel, the coolant channel adapted to receive aheat conductive liquid wherein the facet includes a metal backingcoupled to a metal substrate of the mirror, the metal backing and themetal substrate having alternating projections extending into thecoolant flow channel; and an adhesive compound bond between the backsurface of the mirror and the facet such that the mirror is fixed to thefacet and the facet imposes a compressive stress that depends uponthermal contraction of the facet along the back surface of the mirror sothat tensile stress on the mirror is minimized when the mirror is beingoperated at an operating temperature.
 20. The reflector of claim 19,wherein the reflector further includes a stiffening plate disposedbetween the mirror and the facet.
 21. The reflector of claim 19, whereinthe metal backing of the facet comprises a serpentine fluid flow path.